DECOMPOSITION OF NITRIC-OXIDE OVER BARIUM OXIDE-SUPPORTED ON MAGNESIUM-OXIDE .2. IN-SITU RAMAN CHARACTERIZATION OF PHASES PRESENT DURING THE CATALYTIC REACTION

High temperature in situ Raman spectroscopy was used to determine the complex phase behavior during the decomposition of Ba(NO3)(2) supporte d on MgO. The starting material, crystalline Ba(NO3)(2), is stable up to 500 degrees C in a 1% NO in He gas mixture. Above this temperature or at lower NO partial pressures, the crystalline Ba(NO3)(2) is transf ormed into an amorphous intermediate phase II' which contains nitrate and nitrite ions. This phase is gradually converted into phase II '' c ontaining nitrate ions, Ba-nitrito complexes, and the first traces of a Ba-nitro species. The next step in the decomposition is the formatio n of phase III, which consists mainly of Ba-nitro complexes and nitrat e ions. Phase III is stable in decreasing NO pressures until its decom position into defect-rich BaO, phase IV. A hysteresis was observed for the stability range of phase LII as a function of decreasing or incre asing NO partial pressures. The transformations between phase III and IV are characterized by isosbestic points in the Raman spectra. Thus, decomposition/reformation of phase III occurs directly without any det ectable intermediates. The effect of temperature on the decomposition of phase Ill into the defect-rich BaO, as determined by Raman spectros copy, is in good agreement with the calculated coexistence of BaO, BaO 2, and Ba(NO3)(2) phases. Moreover, there is a good correlation betwee n the presence of phase III and the maximum catalytic activity for the decomposition of NO. These in situ. Raman results strongly suggest th at the observed activity of highly loaded Ba/MgO catalysts arises from the formation of phase III. The sharp falloff in the N-2 formation ra te at a particular temperature is related to the transition between ph ase III and phase N. Transient in situ Raman experiments indicate that Ba-nitro complexes in phase III are taking part in the catalytic cycl e as reaction intermediates.